A spatial data clearinghouse is an electronic facility for searching, viewing, transferring, ordering, advertising, and disseminating spatial data from numerous sources via the Internet. Governments and other institutions have been implementing spatial data clearinghouses to minimise data duplication and thus reduce the cost of spatial data acquisition. Underlying these clearinghouses are geoportals and databases of geospatial metadata. A geoportal is an access point of a spatial data clearinghouse and metadata is data that describes data. The success of a clearinghouse’s spatial data discovery system is dependent on its ability to communicate the contents of geospatial metadata by providing both visual and analytical assistance to a user. The model currently adopted by the geographic information community was inherited from generic information systems and thus to an extent ignores spatial characteristics of geographic data. Consequently, research in Geographic Information Retrieval (GIR) has focussed on spatial aspects of web-based data discovery and acquisition. This thesis considers how the process of GIR can be enhanced through multidimensional visualisation served by web-based geographic data sources. An approach is proposed for the presentation of search results in ontology-assisted GIR. Also proposed is an approach for the visualisation of multidimensional geographic data from web-based data sources. These approaches are implemented in two prototypes, the Geospatial Database Online Visualisation Environment (GeoDOVE) and the Spatio-Temporal Ontological Relevance Model (STORM). A discussion of their design and implementation is presented. Their integration into an existing geoportal is considered, followed by an evaluation of the visualisation prototypes and their usability once deployed on the geoportal. The implications of this research on national spatial data clearinghouses are then considered.